System and method of anodized aluminum oxide nano-porous membrane preparation

ABSTRACT

A system and method for preparation of nano-porous membrane using anodized aluminium oxide and the membrane/film/thin lamina produced thereof. The system comprises a template forming device that comprises of two rolls provided with one or plurality of projections wherein the Al sheet is passed through the said rolls that are rotatable in opposite direction with respect to each other wherein in operation as the Al sheet is passed through the said rolls, the said projections of the rolls punch depressions to the predetermined depth in the said sheet wherein the depth of the depression is governed by the height of the projections. A method for preparation of anodized aluminum oxide nano-porous membrane comprising electro polishing of Al substrate; first step anodization; chemical etching of alumina; second stage anodization; etching Al for separation of alumina and barrier layer removal or voltage pulse detachment for barrier layer removal and detachment of membrane from Al substrate.

FIELD OF INVENTION

The present invention relates to a system and a method for preparation of nano-porous membrane using anodized aluminium oxide and the membrane/film/thin lamina produced thereof.

BACKGROUND OF THE INVENTION

Anodized Aluminum Oxide (AAO) membranes are gaining popularity in the field of sensors, hemodialysis, protein separation, photovoltaics, high density memories, control molecular release, drug release, nano dots, nano rods, nano tubes, and making masks for semiconductors industry. The vital aspects to characterize AAO membrane include pore size, pore depth, inter-pore distance, thickness of membrane and pore geometry.

The method of fabrication of the AAO membrane is reported in the prior art.

U.S. Pat. No. 3,850,762 discloses fabrication of AAO free standing membrane wherein steps include anodizing one side of Al sheet, etching Al substrate for exposing back side of alumina, and then etching of barrier layer (BL) for making AAO membrane. The O-ring setup is used for anodizing one side and then etching Al and BL from other side.

Another reported method includes anodizing Al sheet from both sides and then using O-ring setup for etching alumina from one side, then Al etching and etching BL of other side AAO so that free standing AAO can be achieved.

However, these methods suffer from the drawback that the area of AAO free standing membrane actually produced is substantially lower than that of Al sheet used for fabrication leading to wastage of material. Depending on application low grad Al or high purity Al can be used in the process. If 99.999% pure Al is used in the fabrication of AAO membrane then wastage of material cause vital problem. The 99.999% pure Al is much expensive leading to increasing the cost of the membrane.

There is another method in which Al sheet is painted from one side by insulting material and anodization is carried on other side, after anodization the insulting material on back side is etched chemically, then etching Al and removing BL for making free standing AAO membrane. However, the limitation is that two extra steps of depositing insulting layer and then again removing this layer are required.

PCT patent GB95/01646 discloses electrochemical etching and detachment of AAO from Al substrate. Accordingly the anodized aluminum is subjected to dissolving electrolyte and applying some potential. The dissolving electrolyte opens BL and detaches AAO from Al substrate. This method was further modified by Yuan et al. and named as one step pulse voltage detachment. In this process anodized Al is subjected to electro polishing solution and a voltage pulse is applied for few seconds and it causes to etch BL and detach AAO membrane from Al substrate. This method has a capacity to use for detachment of AAO from both sides of Al sheet in few seconds. Technology has to develop so that such a method can be use for producing more AAO with less amount of Al.

However, it is to be noted that many of applications demand small size of AAO membrane. The limitation of the above mentioned methods result in fabrication of AAO plane membrane in one piece at a time. It is necessary to have plurality of setups for fabrication of number of membrane pieces leading to substantially increasing production and process cost. There is need to develop a method to fabricated plurality of membranes at the same time out of a single Al sheet or block. Technology also has to be developed to produce number of small AAO pieces from a single Al sheet or block.

The tubular membrane is characterized as a three dimensional type of AAO membrane. Tubular membrane is one of 3D membrane which has been used in the field of drug delivery, hemodialysis and gas separation (Gong, 2003; Attaluri, 2009; Li, 2008). N. Itoh et al (Itoh, 1996), fabricated AAO nonporous tubular membrane using one step MA. Later the tubular AAO membranes were fabricated using two step anodization (Gong, 2003; Belwalkar, 2008; Li, 2008; Zhou, 2009). Various attempts have been made to fabricate AAO tubular membrane (Gong, 2003; Belwalkar, 2008; Itoh, 1996; Li, 2008; Zhou, 2009).

The conventional method includes steps to insulate the Al tube from outside, followed by anodization at inner side, and etching Al chemically from outside after removing the insulating cover. Further, the barrier layer is removed using chemical etching. This process can only be used for the manufacture of centimeter and/or millimeter size tubular membrane. However, this process is not appropriate for the manufacture of tubular membranes in micrometer and below size. It is a challenge to make Al tube in small size, covering and uncovering the outer surface for different steps, blocking the two ends to protect AAO from inside during etching process and again opening the two ends to give final shape.

Bong-Young Yoo et. al report fabrication of the micrometer size AAO tubular membrane by drilling in Al block. But most of the steps were same as in previous reported methods (Gong, 2003; Belwalkar, 2008; Itoh, 1996; Li, 2008; Zhou, 2009). In the method proposed by Bong-Young Yoo et al (Yoo, 2006), alumina was removed mechanically from one side of Al block. However this imposes serious limitation in case of nanostructure membranes.

One of the challenges in the development of aluminum oxide membrane is the issue of cracks that occur during the conversion of alumina from aluminum by virtue of ion exchange process. Cracks occur inside AAO and/or at the edges of AAO. Cracks inside AAO normally occur due to rapid growth of AAO as in hard anodization. Woo Lee et al. introduced modified form of hard anodization in which cracks were minimized. To overcome crack problem Woo Lee et al also introduced pulse anodization. In mild anodization crack inside AAO do not occur if appropriate processing conditions are applied. Another type of cracks occurs at the edges wherein aluminum (Al) surfaces are substantially perpendicular to each other. These cracks are due to volume expansion when Al is converted into AAO. This type of cracks made obstacles in preparation of AAO 3D membranes. However attempts are available to utilize cracks as well.

The aspect of utilization of cracks to fabricate anodic aluminum oxide nanoporous tubular and rectangular membrane is reported by Kasi et. al. However, this method suffers disadvantage due to the lack of development of the shaping aspect of Al and relevant production mass production process.

The methods provided in prior art suffer from following limitations:

-   -   Substantially reduced area of AAO free standing membrane         produced from available Al sheet used for fabrication leading to         wastage of material and reduced productivity;     -   Need to insulate Al from any side in the electrolyte solution or         cutting any surface mechanically;     -   Necessity of two extra steps of depositing insulting layer and         then again removing this layer;     -   It is necessary to have plurality of setups for fabrication of         number of membrane pieces leading to substantially increasing         production and process cost.     -   There is need to develop a method to fabricated plurality of         membranes at the same time out of a single Al sheet or block     -   Only single surface anodization of the substrate (rectangular or         square) and removal of entire substrate leading to material         waste, increase in specific energy consumption, substantially         reduced productivity, posing limitation on scaling of the         process/production     -   Lack of utilization of all the surfaces of the substrate to form         the membrane;     -   Need of Al solid cylinder to manufacture tubular membrane,     -   Lack of flexibility and possibility to manufacture tubular         member from rectangular/square, circular geometry substrate;     -   Lack of flexibility and possibility to manufacture tubular as         well as laminar membrane from the same rectangular/square         substrate.

There is a need in the market place to provide a method to enable:

-   -   Fabricating/producing plurality of millimeter or micrometer size         membranes at the same time out of a single Al substrate (sheet         or block);     -   relatively simple method obviating number of complex steps;     -   substantially reducing Al substrate wastage;     -   obviate need of separate set up and method for two dimensional         as well as three dimensional membrane manufacture/production;     -   Flexibility of producing diverse shapes of the membrane such as         tubular, rectangular, conical, fish mouth shaped etc.     -   effective utilization of cracks to produce plurality of         membranes from faces of the Al block/substrate at the same time         in a single process step to obviate material wastage, enhance         productivity and substantially reduce process cost;     -   effective utilization of cracks to produce three dimensional         tubular membrane from Al block;     -   effective utilization of mild anodization (MA), hard anodization         (HA) and pulse anodization;     -   flexibility of detachment of AAO using chemical etching or one         step voltage pulse detachment;     -   fabrication of plurality of millimeter or micrometer size         membranes at the same time out of a single Al sheet or block         obviating the problems of the conventional method.

SUMMARY OF THE INVENTION

The main object of the invention is to provide a system and a method for preparation of nano-porous membrane using anodized aluminium oxide.

Another object of the invention is to provide a method for effective utilization of cracks to produce plurality of membranes from faces of the Al block/substrate at the same time in a single process step to obviate material wastage, enhance productivity and substantially reduce process cost;

Another object of the invention is to provide a method for producing plurality of membranes at the same time out of a single Al substrate (sheet or block) in a single process step.

Yet another object of the invention is to provide a method for producing millimeter or micrometer size membranes at the same time out of a single Al substrate (sheet or block) in a single process step.

Yet another object of the invention is to provide a simple method, preferably single stage method obviating number of complex steps;

Yet another object of the invention is to provide a method to substantially reduce Al substrate wastage.

Yet another object of the invention is to provide a method to obviate need of a separate set up for the manufacture of two dimensional as well as three dimensional membranes.

Yet another object of the invention is to provide a method to enable flexibility of producing diverse shapes of the membrane such as tubular, rectangular, conical, fish mouth etc.

Yet another object of the invention is to provide a method for effective utilization of cracks to produce three dimensional tubular membranes from Al block.

Yet another object of the invention is to obviate problem of cracks inside AAO.

Yet another object of the invention is to provide a method for effectively utilizing a combination of mild anodization (MA), hard anodization (HA) and pulse anodization.

Yet another object of the invention is to provide a method to enable flexibility of detachment of AAO using one step voltage pulse detachment.

Thus in accordance with the invention the system comprises of a template forming device that comprises of two rolls provided with one or plurality of projections wherein the Al sheet is passed through the said rolls that are rotatable in opposite direction with respect to each other where in operation as the Al sheet is passed through the said rolls, the said projections of the rolls punch/create depressions to the desired depth in the said sheet wherein the depth of the depression is governed by the height of the projection

wherein the method comprises of

-   -   electro polishing of Aluminum substrate (Al) comprising steps         of:         -   placing the said Al sheet with the punched depressions in             the mixture/solution of perchloric acid and ethanol             respectively wherein the ratio in the range of 1:3 to 1:5 by             volume wherein purity of ethanol is in the range of             99%-99.9% and that of Perchloric acid is in the range of             69-72%;         -   Applying potential at a temperature less than 10° C. wherein             the potential is in the range of 10 to 20 V;         -   Applying potential for 3 to 10 min depending on the surface             roughness;     -   First step anodization comprising steps of:         -   selecting electrolyte from either of oxalic acid, phosphoric             acid, sulfuric acid and malunic acid wherein the             concentration of the said acid depends on the pore size;         -   using oxalic acid as electrolyte in the range of 0.2M to             0.3M;         -   applying a potential in the range of 35 to 45V wherein the             process time is in the range of 1 to 6 h.     -   chemical etching of the anodized aluminum oxide comprising steps         of:     -   etching in chromic acid and phosphoric acid wherein the         temperature is in the range of 65-80° C. wherein phosphoric acid         is in the range of 6 wt % to 7 wt % and chromic acid is in the         range of 2 wt % to 3 wt % wherein purity of Chromic acid is 99%         and purity of phosphoric acid is 85%;     -   second step anodization comprising steps of:         -   Repeating the process in the first step anodization wherein             hexagonally arranged nanoporous structures are formed with             one end blocked with barrier layer wherein process time             depends on the membrane thickness, it can range from 1 h to             48 h.     -   barrier layer (BL) removal either by chemical etching or voltage         pulse method.     -   wherein chemical etching of barrier layer comprises of steps         -   etching Al in saturated mercuric chloride so as to separate             anodized aluminum oxide from Al;         -   placing of AAO in 5 wt % to 6 wt % Phosphoric acid for about             35 to 40 min at 31° C. to 32° C. for etching of BL.     -   alternately the barrier layer is removed using voltage pulse         method comprising steps of;         -   placing the said substrate in perchloric acid and ethanol             with volume ratio in the range of 1:3 to 1:5 respectively;         -   applying a voltage pulse from 45 to 50 V for 3 to 5 seconds             that causes to detach AAO from Al and remove BL.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of this invention will become apparent in the following detailed description and the preferred embodiments with reference to the accompanying drawings. The embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings.

FIG. 1 illustrates elevation and top view of the system of the present invention.

FIG. 2 illustrates the schematic of the punched Al sheet as one of the embodiments.

FIG. 3(A) to (C) illustrates schematically the formation of Anodized Aluminum Oxide (AAO) and the aspect of occurrence of cracks.

FIG. 4 illustrates one of the embodiments of punching circular outline in the Al sheet FIG. 5(A) to (B) illustrates one of the embodiments relating to two and three dimensional membrane preparation from Al block.

FIG. 6(A) to (F) illustrates the formation of the one dimensional membranes using cracks from the Al block FIG. 7(A) to (D) illustrates SEM images during the process of preparation of three dimensional membrane FIG. 8(A) to (H) depicts membrane characterization during each of the stage of membrane preparation.

FIG. 9 depicts the SEM image illustrating hexagonal order of the pores

DESCRIPTION OF THE INVENTION

In the following description, various embodiments will be disclosed. However, it will be apparent to those skilled in the art that the embodiments may be practiced with only some or shall disclosed subject matter. For purposes of explanation, specific numbers, materials, and/or configuration are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without one or more of the specific details, or with other approaches, materials, components etc. In other instances, well-known structures, materials, and/or operations are not shown and/or described in detail to avoid obscuring the embodiments. Accordingly, in some instances, features are omitted and/or simplified in order to not obscure the disclosed embodiments. Further more, it is understood that the embodiments shown in the Figures are illustrative representation and are not necessarily drawn to scale.

The system of the present invention comprises of a template forming system depicted in FIG. 1 that illustrates elevation and top view of the system. It comprises of first roll 1 and second roll 2. Each of the said roll is provided with at least one projection indicated as 1 a, 1 b and 2 a, 2 b as illustrated in the FIG. 1. Al sheet 3 is passed through the said rolls in the direction indicated by the arrow 5. The said first and second rolls are rotatable in opposite directions so as to punch the depression 4 in the said Al sheet from top and bottom surface of the sheet as indicated in FIG. 1.

One of the embodiments of such punched Al sheet is depicted in FIG. 2. The punched lines with depression can be created in horizontal and vertical manner indicated by lines with numeral 21 and 22 respectively in the FIG. 2. Part of the Al sheet 23 is extended to be used for connection with the electrode in the anodization process. The pattern of the punched lines with horizontal and vertical combination can be designed according to the dimension of the membrane required.

In one of the embodiments the said depressions can also be created using a sharp tool. In one of the embodiments the Al sheet is a block or piece of a block etc. The person trained in the art can contemplate various forms of Al geometry. The FIGS. 1 and 2 are represent one of the preferred embodiments of the invention, however the process step is not restricted to merely Al sheet.

The nano-porous membrane is produced from such an Al sheet provided with depressions as illustrated in FIG. 3. The method comprises of:

(i) electro polishing of AI substrate (ii) first step anodization; (iii) chemical etching of the alumina; (iv) second stage anodization; (v) etching Al for separation of alumina and barrier layer removal

-   -   or voltage pulse detachment for barrier layer removal and         detachment of membrane from Al substrate

The process of electro polishing of Al sheet comprises steps of:

-   (i) placing the said Al sheet with the punched depressions in the     mixture of perchloric acid and ethanol respectively wherein the     ratio of the perchloric acid and ethanol is in the range of 1:3 to     1:5 by volume wherein purity of ethanol is in the range of 99%-99.9%     and that of perchloric acid is in the range of 69-72%; -   (ii) Applying potential at a temperature less than 10° C. wherein     the potential is in the range of 10 to 20 V; -   (iii) Applying potential for 3 to 10 min depending on the surface     roughness.

The electro polished sheet of Al as depicted in FIG. 3( a) is further anodized. The process of first stage anodization comprises steps of:

-   (i) selecting electrolyte from either of oxalic acid, phosphoric     acid, sulfuric acid and malunic acid wherein the concentration of     the said acid depends on the pore size; -   (ii) using 0.3M oxalic acid as electrolyte; -   (iii) applying a potential in the range of 35 to 45V wherein process     time depends on the membrane thickness, it can range from 1 h to 48     h.

The process of the chemical etching of the anodized aluminum oxide substrate comprises steps of:

-   -   performing chemical etching in chromic acid and phosphoric acid         wherein the temperature is in the range of 65-80° C. wherein         preferably 6 wt % phosphoric acid is 6 wt % and chromic acid is         2 wt % wherein purity of Chromic acid is 99% and purity of         phosphoric acid is 85%;

The second stage anodization process comprises steps of as elaborated in the first stage anodization. This process results in the formation of hexagonally arranged nanoporous structures with one end blocked with barrier layer.

As depicted in FIG. 3 b, the process of anodization results in formation of Anodized Aluminum Oxide (AAO) layer 32 on the Al substrate 31. It is to be noted that during the anodization process and formation of AAO there is expansion of the substrate resulting in displacement of the surface of the AAO due to increase in volume. This results in occurrence of cracks at edges wherein aluminum (Al) surfaces are substantially perpendicular to each other. This phenomenon is illustrated in FIG. 3. The cracks 33 occur at the edges as depicted therein. However it is to be noted that as depicted in FIG. 4 it is possible to punch circular outline wherein there is as well occurrence of crack at the edges wherein the surfaces are substantially perpendicular.

It is surprisingly found that these cracks can be utilized effectively as a passage/conduit to reach the chemical at the Al substrate surface for etching purpose. There is occurrence of cracks at every edge of the substrate. The present invention utilizes these cracks as a means to pass on the chemical upto the each of the surface of the substrate so as to etch the AAO from that surface. It is to be noted that the substrate is entirely immersed in the chemical (saturated mercuric chloride) during chemical etching process and NOT that only one surface is etched as practiced in the conventional method. The immersion of the entire surface in the chemical for chemical etching results in the AAO layer detachment from each of the faces of the substrate. Thus in the present depiction the substrate is in the form of rectangular/cubical geometry having six surfaces wherein each of the surface would then yield an AAO layer after chemical etching. However since one surface is used as electrode, practically there would be five such AAO layer formation from each substrate of rectangular or cubical geometry. This obviates problems associated with insulating the substrate and enables effective utilization of all the surfaces resulting in substantial reduction in material wastage.

The Al sheet/substrate with AAO layer and cracks is immersed in saturated mercuric chloride for chemical etching so as to separate anodized aluminum oxide (AAO) from Al. Thus each of the surface on which AAO is formed is utilized to produce an AAO membrane with barrier layer (BL).

The barrier layer is removed either by chemical etching or voltage pulse method. The method of chemical etching comprises steps of:

-   -   etching Al in saturated mercuric chloride so as to separate         anodized aluminum oxide from Al;     -   placing of AAO in 5 wt % to 6 wt % Phosphoric acid for about 35         to 40 min at 31° C. to 32° C. for etching of BL.

Alternately the barrier layer is removed using voltage pulse method that comprises steps of:

-   -   placing the said substrate in perchloric acid and ethanol with         volume ratio in the range of 1:3 to 1:5 respectively;     -   applying a voltage pulse from 45 to 50 V for 3 to 5 seconds that         causes to detach AAO from Al and remove BL.

The present invention enables manufacture/preparation of two dimensional as well as three-dimensional membrane simultaneously in the same process step using a single substrate. One of the embodiments is depicted in FIG. 5. The FIG. 5 (a) illustrates the Al substrate 50 in the form of a block provided with electrode end 51. One or plurality of holes 52 are drilled in the said block 50 as indicated in the FIG. 5( a). There are five surfaces available in the said substrate 50. The front and back surface 53 and 54 respectively wherein holes are drilled; top and bottom surface 55 and 56 respectively and the side surface 57 as depicted in the FIG. 5( a).

The said substrate (drilled block 50) is processed using the method of electro polishing, first step anodization, chemical etching of the anodized Al, second stage anodization, etching for separation of membrane and barrier layer removal. The details of the method are already mentioned above. It is be noted that each of the said surfaces of the said substrate 50 result in the production of the membrane as a result of the method of the present invention. As indicated in FIG. 5( b), the front surface 53 and back surface 54 yield the membranes 53 a and 54 a respectively. The top surface 55 and bottom surface 56 yields membranes 55 a and 56 a. These are all two dimensional membranes. It is to be underlined that the drilled holes 52 in the said block/substrate 50 yield the three dimensional membranes in the form of a tube/s 52 a. The same substrate/block is thus utilized to produce three dimensional as well as two dimensional membranes simultaneously.

In one of the variants of this embodiment, based on the requirement, one can create other than cylindrical forms as well from the said block.

The nano-porous membrane prepared by the method of present invention result in majority of pores with a constant diameter, and inter-pore distance. In one of the embodiments the pore diameter is 37 nm, and inter-pore distance 95 nm which is in great agreement with 10% porosity rule.

In one of the embodiments the AAO membrane is prepared using 99.56% pure Al. In another variant of this embodiment the AAO membrane is prepared using 99.999% pure Al.

The technical effect of this inventive aspect of the present invention is tangible, concrete and measurable. This is quantified and elaborated in the following non-limiting examples

Example 1

As illustrated in FIG. 6( a), Al block 60 is used as a substrate to establish the aspect of the invention of using cracks at the edges to produce membranes from the five faces of the said substrate. One of the ends 61 is used as electrode in the anodization process. The said substrate 60 is treated using method of the present invention comprising steps of electro polishing of Al substrate, first step anodization, chemical etching of the alumina, second stage anodization and etching Al for separation of alumina and barrier layer as elaborated in the description of the invention.

FIG. 6( b) represents schematically the aspect of Al substrate 62 and formation of AAO and development of the crack 64 at the edge. FIG. 6( c) depicts SEM image of the said substrate 60 after anodization wherein cracks 64 at the edges are observed. Further AAO 63 on the surfaces of the said substrate are observed in this SEM image. FIG. 6( d) depicts the magnified image of the AAO surface 63 (exploded view of the rectangular mark indicated on FIG. 6( c), depicting the hexagonal pores structure on the surface of AAO.

FIG. 6( e) illustrates the schematic of the AAO that could be derived from the faces of the said substrate 60. Numerals 65 to 69 respectively depict the AAO from back surface, front surface, top, bottom and side surface of the substrate. FIG. 6( f) depicts SEM image of the AAO separated from one of the said surfaces.

Example 2

Using the method of the present invention, three dimensional nano-porous tubular membranes are obtained.

FIG. 7( a) depicts SEM image of the nano-porous tubular membrane. The FIG. 7( b) depicts image of the top surface plane membrane separated from tubular membrane. The rectangle in the image indicates the cut edge of the top plane membrane magnified image of which is depicted in FIG. 7( c). Further the FIG. 7 (d) indicates the blown up image of the portion indicated by rectangle in FIG. 7( c). The pores 70 of AAO as well as the barrier layer 71 are seen in FIG. 7( d).

Example 3

Using the method of the present invention, nano-porous membranes are obtained. The Al substrate used in this non-limiting example is 99.999% pure. The characterization at each of the process stages is depicted in FIG. 8.

-   -   The surface SEM result of 99.999% pure electro-polished Al is         depicted in FIG. 8( a). The process of electro polishing results         in the removal of micro size roughness from the said Al         substrate surface. However nano size random pits can be seen in         this image. These nano size pits causes the initiation of pores         in first step of anodization.     -   In first step of anodization the AAO forms with random pores as         shown in FIG. 8( b). These pores initiates in nano pits or         concave surfaces on the Al substrate surface. As these pores         propagate inside (in the form of a nano-channel) the substrate         some of the pores proceed and some are blocked. Pores are         further arranged in hexagonal order with substantially equal         distance from each other.

The AAO formed in first step of anodization result in random pore structure at top surface but at bottom of each nano-channel there is substantial periodic configuration/distribution. The AAO formed in the first step of anodization is etched to access the periodic structure on the Al surface. FIG. 8( c) depicts the surface SEM image after etching of the AAO formed in the said first step anodization process. The hexagonal structure can be seen on the Al surface.

-   -   The second step anodization is carried on the said etched Al         surface having periodic hexagonal order of nano pits/concave         surfaces. The membrane formed in the second step of anodization         is of substantially hexagonal order as depicted in FIG. 8( d).         It can be observed that majority of pores is with constant         diameter and have constant inter-pore distance. In this         embodiment the pore diameter is 37 nm, and inter-pore distance         95 nm which is in good agreement with 10% porosity rule.     -   The bottom portion of the formed nano-channel of the AAO is         covered by the barrier layer (BL). It is depicted in the FIG. 8(         e). It is observed that cells are in hexagonal shape and seven         cells combine to make hexagonal order.     -   FIG. 8( f) depicts the cross-section SEM image of AAO formed in         the second step of anodization. Substantially straight and         parallel nano-channels are observed in this image. The diameter         of the said nano-channel is substantially uniform. Further the         distance between the said nano-channels is substantially         uniform.     -   The BL needs to be etched to prepare a nano-porous AAO membrane.         FIG. 8( g) depicts the SEM image of AAO membrane wherein the BL         is etched. It is to be noted that the said process of chemical         etching is used for BL detachment.     -   FIG. 8( h) depicts the SEM image of AAO which was detached from         Al using the said voltage pulse detachment. Here pore diameter         is 37 nm and inter-pore distance 95 nm.

Example 4

In this representative non-limiting example, AAO is prepared using 99.56% pure Al. The AAO is prepared using method of the invention described above. FIG. 9 depicts the hexagonal order of pores. 

We claim:
 1. A system for preparation of anodized aluminum oxide nano-porous membrane comprising a template forming device that comprises of two rolls provided with one or plurality of projections wherein the Al sheet is passed through the said rolls that are rotatable in opposite direction with respect to each other wherein in operation as the Al sheet is passed through the said rolls, the said projections of the rolls punch depressions to the predetermined depth in the said sheet wherein the depth of the depression is governed by the height of the projections.
 2. A system for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 1 wherein the punched lines with depression are created in horizontal and vertical manner wherein part of the Al sheet is extended to be used for connection with the electrode in the anodization process wherein the pattern of the punched lines with horizontal and vertical combination is designed according to the dimension of the membrane required.
 3. A method for preparation of anodized aluminum oxide nano-porous membrane comprising (i) electro polishing of Al substrate (ii) first step anodization; (iii) chemical etching of alumina; (iv) second stage anodization; (v) etching Al for separation of alumina and barrier layer removal or voltage pulse detachment for barrier layer removal and detachment of membrane from Al substrate.
 4. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein the process of electro polishing of Al sheet comprises steps of: (i) placing the said Al sheet with the punched depressions in the mixture of perchloric acid and ethanol respectively wherein the ratio of the perchloric acid and ethanol is in the range of 1:3 to 1:5 by volume wherein purity of ethanol is in the range of 99%-99.9% and that of perchloric acid is in the range of 69-72%; (ii) applying potential at a temperature less than 10° C. wherein the potential is in the range of 10 to 20 V; (iii) applying potential for 3 to 10 min depending on the surface roughness.
 5. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein the process of the first stage anodization comprises steps of: a. selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size; b. using oxalic acid as electrolyte; c. applying a potential in the range of 35 to 45V wherein process time depends on the membrane thickness, it can range from 1 h to 48 h.
 6. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein the process of the chemical etching of the anodized aluminum oxide substrate comprises step of: performing chemical etching in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein preferably 6 wt % phosphoric acid is 6 wt % and chromic acid is 2 wt % wherein purity of chromic acid is 99% and purity of phosphoric acid is 85%.
 7. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein the second stage anodization process comprises steps of a. selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size; b. using oxalic acid as electrolyte; c. applying a potential in the range of 35 to 45V wherein process time depends on the membrane thickness, it can range from 1 h to 48 h.
 8. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein cracks at edges of the substrate wherein aluminum (Al) surfaces are substantially perpendicular to each other are utilized effectively as a passage to reach the chemical at the Al substrate surface to etch the AAO from that surface.
 9. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein the substrate is entirely immersed in saturated mercuric chloride during chemical etching process wherein the immersion of the entire surface in the mercuric chloride results in the AAO layer detachment from each of the faces of the substrate enabling each of the substrate surface except surface used as electrode to yield an AAO layer after chemical etching.
 10. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein the barrier layer is removed comprising steps of: etching Al in saturated mercuric chloride so as to separate anodized aluminum oxide from Al; placing of AAO in 5 wt % to 6 wt % Phosphoric acid for about 35 to 40 min at 31° C. to 32° C. for etching of BL.
 11. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein the barrier layer is removed using voltage pulse method that comprises steps of: placing the said substrate in perchloric acid and ethanol with volume ratio in the range of 1:3 to 1:5 respectively; applying a voltage pulse from 45 to 50 V for 3 to 5 seconds that causes to detach AAO from Al and remove BL.
 12. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein preparation of two dimensional as well as three-dimensional membrane simultaneously in the same process step using a single substrate is carried out wherein the Al substrate (50) in the form of a block provided with electrode end (51); one or plurality of holes (52) are drilled in the said block (50), there are five surfaces available in the said substrate (50); the front and back surface (53 and 54) respectively wherein holes are drilled; top and bottom surface (55 and 56) respectively and the side surface (57); the said Al substrate (drilled block 50) is processed using the method of electro polishing, first step anodization, chemical etching of the anodized Al, second stage anodization, etching for separation of membrane and barrier layer removal; each of the said surfaces of the said substrate (50) result in the production of the two dimensional membranes wherein the drilled holes (52) in the said substrate (50) yield the three dimensional membranes in the form of a tube/s (52 a).
 13. A method for preparation of anodized aluminum oxide nano-porous membrane as claimed in claim 3 wherein substrate is selected from 99.56% pure Al or 99.999% pure Al.
 14. The anodized aluminum oxide nano-porous membrane with pore diameter in the range of 30 to 40 nm, and inter-pore distance in the range of 90 to 100 nm. 